Agricultural products, such as vegetables and flowers, are grown on a large scale in greenhouses throughout the world. During daylight hours, the agricultural products growing in a greenhouse introduce water vapor into the greenhouse and extract carbon dioxide from the air therein. Often, the addition of heat to a greenhouse on a daily basis is required depending on geographical location and season of the year. In some locations, heat is needed only at night.
To reduce the heat load on a greenhouse, it is conventional to seal the greenhouse with plastic film to obtain a reasonably insulated structure at a moderate price. This situation is illustrated in FIG. 1 which shows the provision of a conventional space heater in a greenhouse for the purpose of heating the greenhouse. An improved arrangement is shown in FIG. 2 wherein the greenhouse is doubly insulated with plastic film. Regardless of the degree of insulation, experience shows that in heated greenhouses at night, the plants are often warmer than the ambient air outside the greenhouse, and that the humidity level inside the greenhouse is considerably elevated due to the transpiration rate of the plants even in their quiescent state under conditions of low light intensity. The more securely a greenhouse is sealed, the greater is the enhancement of condensation heat flux to the walls of the greenhouse and the cold portions of the plants. In order to inhibit plant diseases due to high humidity conditions, removal of excess water vapor is achieved by ventilation of the greenhouse during the night. Ventilation of this type usually is carried out periodically. Such conditions also occur during periods of rather cold weather when the light level is low. Often, periodic ventilation is also commonly used during these periods to remove excess water vapor.
During ventilation of a greenhouse, buoyant warm, moist air adjacent the ceiling of the greenhouse is removed through a selectively operable vent in the ceiling, this air being replaced by the colder, more dense outside air that flows toward the floor and the plants in the greenhouse. The humidity inside the greenhouse is thus reduced, but at a cost of a reduction in temperature inside the greenhouse. To maintain a suitable temperature for the growing plants, heaters are provided and operated to return the internal temperature of the greenhouse to a suitable level. Often, heating of the enclosure air as well as venting is repeated cyclically during the night-time or such periods as suggested in FIG. 4 which is a highly idealized time chart showing the cyclical heating and cooling of the interior of such a greenhouse. In the chart in the upper portion of FIG. 4, the operation of the heater is shown; and, the chart in the lower portion of FIG. 4 shows the resultant cyclical changes in humidity and temperature inside the greenhouse on the basis that ventilation occurs periodically.
From actual experience, the heat load on the greenhouse is not inconsequential. This can be appreciated by considering that the moist air removed during ventilation contains a considerable amount of heat that is lost to the atmosphere: the latent heat contained in the vented vapor, and the sensible heat in the air. This lost heat must be replaced by a heater.
The heat load on the greenhouse can be reduced by utilizing direct contact air-brine-vapor heat exchangers in the greenhouses for the purpose of reducing humidity and which may be used in the manner described below. During the day, brine in the heat exchanger will be hygroscopic because the vapor pressure at the air/brine interface at a given temperature will be less than the vapor pressure of vapor in the air at the same temperature. This approach to a more efficient use of the latent condensation of the vapor in a greenhouse is disclosed in U.S. Pat. No. 4,707,995 granted Nov. 24, 1987, and in U.S. Pat. No. 4,819,447 granted Apr. 11, 1989, the subject matter of these patents being hereby incorporated by reference. These patents disclose temporarily storing, in the brine present in the enclosure, latent heat liberated by the condensation of vapor on the brine, and contacting the air in the enclosure with the brine during the night when the air in the enclosure normally will be cooler than the brine. Heat extracted from the greenhouse during the day and temporarily stored in the brine is thus transferred, at night, back to the air in the enclosure by a reduction in the sensible heat of the brine. Such an approach can also be used during periods of rather cold weather and also during periods of low light levels.
While this approach provides a reduction in the heat load on the greenhouse, it also raises the possibility that the brine used in the greenhouse at night will be at a temperature such that the brine will not be hygroscopic. As a consequence, while the heat extracted from the greenhouse during the day by the condensation of water vapor on concentrated brine is stored and returned to the enclosure at night, additional water vapor may be introduced into the greenhouse at night by the evaporation of water from the brine. In such a situation, ventilation at night would be required to also remove such extra water vapor and to maintain the humidity in the greenhouse at a level which inhibits various types of plant diseases.
Much effort has been expended in efforts to reduce the heat loads in greenhouses occasioned by the necessity for their ventilation. It is therefore an object of the present invention to provide a new and improved method of an apparatus for reducing the heat load on a greenhouse, particularly during periods of low light levels.